Why didn't maryrose cooperate?

Hey so i was wondering, i started watching Shadow's house recently and i'm at season 2 episode 9, i was wondering, why didn't maryrose propose a cooperation between her kate and john, since she doesn't have as much power, but can instead use the power of others for her own, couldn't she have made a bridge, made of ghosts across the cliff? Given the enormous amount of soot, both john and Kate have, i think it could've worked out, maybe they could recruite some more if it's not enough, or maybe even used some of th soot, seeping out of the constructions.

But those are just my thoughts, Thank you.